Definitions

# Central carrier

In the context of von Neumann algebras, the central carrier of a projection E is the smallest central projection, in the von Neumann algebra, that dominates E. It is also called the central supporting projection or central cover.

## Definition

Let L(H) denote the bounded operators on a Hilbert space H, ML(H) be a von Neumann algebra, and M` the commutant of M. The center of M is Z(M) = M`M = {TM | TM = MT for all MM}. The central carrier C(E) of a projection E in M is defined as follows:

C(E) = ∧ {FZ(M) | F is a projection and FE}.

The symbol ∧ denotes the lattice operation on the projections in Z(M): F1F2 is the projection onto the closed subspace generated by Ran(F1) ∩ Ran(F2).

The abelian algebra Z(M), being the intersection of two von Neumann algebras, is also a von Neumann algebra. Therefore C(E) lies in Z(M).

If one think of M as a direct sum (or more accurately, a direct integral) of its factors, then the central projections are the direct sums of identity operators in the factors. If E is confined to a single factor, then C(E) is the identity operator in that factor. Informally, one would expect C(E) to be the direct sum of identity operators I where I is in a factor and I · E ≠ 0.

## An explicit description

The projection C(E) can be described more explicitly. It can be shown that the Ran C(E) is the closed subspaces generated by MRan(E).

If N is a von Neumann algebra, and E a projection that does not necessarily belong to N and has range H`. The smallest central projection in N that dominates E is precisely the projection onto the closed subspace [N`H`] generated by N`H`. In symbols, if

F' = ∧ {FN | F is a projection and FE}

then Ran(F`) = [N`H`]. That [N`H`] ⊂ Ran(F`) follows from the definition of commutant. On the other hand, [N`H`] is invariant under every unitary U in N`. Therefore the projection onto [N`H`] lies in N. Minimality of F` then yields Ran(F`) ⊂ [N`H`].

Now if E is a projection in M, applying the above to the von Neumann algebra Z(M) gives

Ran C(E) = [Z(M)` Ran(E) ] = [(M`M)` Ran(E) ] = [MRan(E)].

## Related results

One can deduce some simple consequences from the above description. Suppose E and F are projections in a von Neumann algebra M.

Proposition ETF = 0 for all T in M if and only if C(E) and C(F) are orthogonal, i.e. C(E)C(F) = 0.

Proof:

ETF = 0 for all T in M.
⇔ [M Ran(F)] ⊂ Ker(E).
C(F) ≤ 1 - E, by the discussion in the preceding section, where 1 is the unit in M.
E ≤ 1 - C(F).
C(E) ≤ 1 - C(F), since 1 - C(F) is a central projection that dominates E.
This proves the claim.

In turn, the following is true:

Corollary Two projections E and F in a von Neumann algebra M contain two nonzero subprojections that are Murray-von Neumann equivalent if C(E)C(F) ≠ 0.

Proof:

C(E)C(F) ≠ 0.
ETF ≠ 0 for some T in M.
ETF has polar decomposition UH for some partial isometry U and positive operator H in M.
Ran(U) = Ran(ETF) ⊂ Ran(E). Also, Ker(U) = Ran(H) = Ran(ETF) = Ker(ET*F) ⊃ Ker(F); therefore Ker(U))Ran(F).
⇒ The two equivalent projections UU* and U*U satisfy UU*E and U*UF.

In particular, when M is a factor, then there exists a partial isometry UM such that UU*E and U*UF. Using this fact and a maximality argument, it can be deduced that the Murray-von Neumann partial order « on the family of projections in M becomes a total order if M is a factor.

Proposition (Comparability) If M is a factor, and E, FM are projections, then either E « F or F « E.

Proof:

Let ~ denote the Murray-von Neumann equivalence relation. Consider the family S whose typical element is a set { (Ei, Fi) } where the orthogonal sets {Ei} and {Fi} satisfy EiE, FiF, and Ei ~ Fi. The family S is partially ordered by inclusion and the above corollary shows it is non-empty. Zorn's lemma ensures the existence of a maximal element { (Ej, Fj) }. Maximality ensures that either E = ∑ Ej or F = ∑ Fj. The countable additivity of ~ means Ej ~ ∑ Fj. Thus the proposition holds.

Without the assumption that M is a factor, we have:

Proposition (Generalized Comparability) If M is a von Neumann algebra, and E, FM are projections, then there exists a central projection PZ(M) such that either EP « FP and F(1 - P) « E(1 - P).

Proof:

Let S be the same as in the previous proposition and again consider a maximal element { (Ej, Fj) }. Let R and S denote the "remainders": R = E - ∑ Ej and S = F - ∑ Fj. By maximality and the corollary, RTS = 0 for all T in M. So C(R)C(S) = 0. In particular R · C(S) = 0 and S · C(S) = 0. So multiplication by C(S) removes the remainder R from E while leaving S in F. More precisely, E · C(S) = (∑ Ej + R) · C(S) = (∑ Ej) · C(S) ~ (∑ Fj) · C(S) ≤ (∑ Fj + S) · C(S) = F · C(S). This shows that C(S) is the central projection with the desired properties.

## References

• B. Blackadar, Operator Algebras, Springer, 2006.
• S. Sakai, C*-Algebras and W*-Algebras, Springer, 1998.
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